1. Field of the Invention
The invention relates to a delay device and the use thereof in the decoding device of distances measuring equipment.
2. Description of the Prior Art
A distance measuring system, forming the subject of the present invention, or D.M.E. for Distance Measuring Equipment--is equipment for aiding air navigation, comprising interrogator equipment placed on board an aircraft (airplane, balloon or helicopter, for example) and transponders placed on the ground and whose role is to measure the oblique distance between the aircraft in flight and the DME stations on the ground. This DME system belongs to the polar coordinate navigational system, whose standards have been standardized by the International Civil Aviation Organization (I.C.A.O).
The operating principle of a DME system is the following. The interrogator equipment placed on board the aircraft sends interrogations signals to the ground stations, in the form of pulse pairs spaced apart by a time T, thus defining the interrogation code. The ground stations receive the interrogations which are intended for them without having to know their geographical origin and send back synchronous responses to the interrogation, also in the form of pulse pairs characterized by a response code T'. By measuring the delay t between the transmission of the interrogation by the equipment, on board the aircraft, and the reception of the response, transmitted by the ground station, the equipment on board the aircraft calculates the oblique distance between the aircraft and the ground station (FIG. 1).
The delay has two components:
the propagation time of the pulse pairs between the aircraft and the ground and vice versa; PA1 a systematic delay .tau. due to the ground transponder and fixed at a constant value for a given station, so that the information received by the aircraft has time to be processed. Thus, the oblique distance D between the aircraft and the ground station is given by the formula: ##EQU1## with t=the time elapsing between the interrogation transmitter by the aircraft and the response received by the ground station, and c=the speed of propagation of the information. PA1 if the interrogation is made over the channel which is allocated to it and PA1 if the code of the interrogation, that is to say the spacing between two pulses, corresponds to the operating mode programmed in this station. PA1 .+-.200 ns for N type interrogations and PA1 .+-.30 ns for P type interrogations and the variations of the delay R.sub.c required for the decoding time and which must be programmable as a function of the mode of the station, on the other hand: PA1 12 .mu.s for mode X, type N and PA1 42 .mu.s for mode Y, type P the problem then consists in providing an accurate delay of PA1 .+-.30 ns which may reach 42 .mu.s in mode Y, type P. PA1 means for acquiring the electric signal, sampling it at a given frequency f.sub.e and delivering it in parallel in the form of M bit binary words, at a frequency f.sub.m such that: EQU f.sub.m :=f.sub.e /M PA1 means for storing the signal in the form of binary words, operating at the frequency f.sub.m and storing each binary word of M bits for a given period of time, PA1 means for restoring, at frequency f.sub.e, the electric signal from M bit binary words read out in parallel from the storage means.
The subject matter of the application relates to the transponder situated in the ground DME station. According to the I.C.A.O. standards, the frequency bands used for transmissions in a DME system is between 962 MHz and 1213 MHz, subdivided into 1 MHz channels, allocated to the different ground DME stations. In addition to this transmission over a particular channel, each transponder may operate in four different modes, called modes X, Y, W or Z and themselves each characterized both by a given code for the interrogations and the corresponding code for the responses and by a given systematic delay .tau.. FIG. 2 shows the specification table for the different operating modes of a DME transponder according to the ICAO standards.
Channels X and Y are used by the DME-N system, also called "en route" system. Channels X, Y, Z, W are used by the DME-P precision system which are essentially associated with the ILS landing system. The DME-P interrogation device in the approach mode uses, depending on the distance D measured on board, the N or P modes (distance D greater than 7 nautical miles for mode N and distance D less than 7 nautical miles for mode P) in accordance with FIG. 2 and changes the coding of the interrogation pulses as a function of the selected mode. This double interrogation mode system proposed by THOMSON-CSF has been adopted by the AVOP of ICAO as a general basis for operation of the DME-P system.
Thus, an aircraft desiring to known its oblique distance with respect to a given ground DME station, will transmit pulses pairs for interrogating this station. The ground DME station will only reply if it recognizes as valid the interrogation which it receives, in other words:
The accuracy obtained by the DME system depends on the type of interrogation chosen by the aircraft. When the aircraft is in the interrogation mode up to 200 nautical miles or more around the ground beacon, it uses interrogations of the "normal" type --N-- to which the ground station will reply with responses of the same type (i.e., N). The oblique distance information between the ground station and the aircraft will be given with a precision of 0.25 nautical miles. On the other hand, if the aircraft is in the landing mode in a zone less than 7 nautical miles about the ground station, the accuracy with which the oblique distance information will be given will be better, of the order of 30 meters, by using interrogations of the "precision" type-P.
FIG. 3 shows the block diagram of a DME transponder placed in a ground station and whose functional description follows. According to the foregoing the DME transponder receives the interrogation pulse pairs from aircraft, decodes the signals received so as to recognize those which it considers as valid i.e. that is to say those which are intended for it, initiates the responses to the interrogation while taking into account the systematic delay, and codes the responses before supplying the transmission signals so as to have the time to process the received interrogations.
For that, the transponder is formed by an antenna 1 which is omnidirectional for an "en route" DME station used for navigating and preferably directional for a "landing" DME station--called DME.P--during landing approach, picking up the interrogations coming from the aircraft; the low or high level signals received with random distribution, pass through a diplexer circuit 2 before passing into a receiver circuit 3 which, after amplification and detection, delivers the interrogation pulse pairs in video form. For proper use of this video information, the pulses pass through three processing circuits 4, 5 and 6 before being recognized valid and delayed by a circuit 7, forming the subject of our application. Finally, the DME transponder comprises a circuit 80 for coding the responses intended for the transmission circuit 8; these responses then pass through the diplexer 2 before being broadcast by the antenna 1.
The video information, at the output of the receiver circuit 3, is formed of a large number of pulses, some corresponding to the valid interrogations, i.e. having the correct spacing corresponding to the ground DME station considered, the others corresponding to the interrogations intended for neighboring ground DME stations not having the same mode and yet others corresponding to multipath echos which may themselves considerably disturb valid interrogations. Since this video information is used for determining the distance between an aircraft and a ground station, it is important to be free of the errors which might arise from any modification that might exist in the interrogation signal received, due to parasites. Therefore, to use this video information as reliably as possible, three circuits are used the purpose of one of which, circuit 4, is to validate the video pulses with respect to the reception frequency. Receiver 3 has an intermediate wide band frequency channel, of the order of 4 MHz, for letting through the pulses having a rapid rising front, which results in taking into consideration several adjacent channels and so mixing the valid interrogations with those of the neighboring channels. Circuit 4 only validates the interrogation received at the correct frequency of the channel corresponding to the ground DME station considered.
The second circuit 5 is a circuit for detecting the arrival time of the pulses, at -6 decibels (dB) below the peak value of the pulses received by the transponder. The output of this circuit is used for processing the N type interrogations and for verifying the coding of the interrogations pulses.
Finally, the third circuit 6 is a circuit for detecting the arrival time of the first pulse of an interrogation pair of the P type. The arrival time of such a pulse must be accurately known and must be free of errors due to the deformation of this pulse by its own echo for it is important for the pilot of the aircraft to know, with the highest possible accuracy, the distance of the aircraft with respect to the ground. So the detection of the arrival time of the first pulse must be made as rapidly as possible before its deformation. For that, a circuit is known used under the name D.A.C.--"Delay and Compare"--the operation of which based on a delay and a comparison is described in the French patent application published under No. 2 497 958 in the name of the applicant corresponding to U.S. Pat. No. 4,518,694.
When the pulses received by the DME transponder have been shaped by the three preceding circuits, the path leads to the dual function circuit 7 which decodes the pulse pairs for identifying the valid interrogations and introduces a delay .tau., which is constant and fixed at a given value for the ground station for processing the information received.
Then, the transponder DME responds to the aircraft by emitting signals formed from pulse pairs, coded by a coder circuit 80 in accordance with the operating mode of the station, and by coded pulses coming from a so called "squitter" circuit whose purpose is to maintain a minimum rate of pseudo-random transmission at the transmitter. The purpose of the "squitter" is essentially to maintain a constant charge rate for the transmitter; for that, if the number of responses to real interrogations decreases the number of pulses pairs generated by the squitter increases and vice versa. Finally, the transmission circuit 8 insures generation of the signals which will be applied to antenna 1.
Since the invention relates to the decoder circuit 7, a general outline the principle for decoding the interrogation pulses received by the DME transponder on the ground and describe the different embodiments of decoders already existing will be provided below.
FIG. 4 shows the general diagram of a decoder 9. The principle of the decoding is as follows: in a circuit 91 the first pulse of the pair received by the transponder is delayed by a time equal to the code to be recognized--the code being represented by the spacing between the first and the second pulse--then the signal obtained at the output of the delay circuit is compared with the non delayed signal; the comparison may be made by means of an AND gate 92. If the pulse pair thus processed presents the correct code, there is coincidence between the second pulse and the first pulse at the output of the delay circuit. To complete the description of the transponder, the decoding principle will be briefly described associated with the systematic delay, the diagram of which is given in FIG. 5. It was mentioned above that the decoding took place from a signal detected at -6 dB below the peak value, this is why the decoder circuit 9 is connected to the output of the -6 dB detection circuit 5. When the decoder circuit 9 has identified the interrogation sent by the aircraft and has recognized it as conforming to the mode of the ground DME station, it validates the pulses at the output of a first compensation delay circuit 10 by means of an AND gate 93 for example. Circuit 10 is connected to the output of the DAC detection circuit 6 which accurately gives the arrival time of the first pulse of the pair serving for the DME-P interrogation. The compensation delay is equal to the time R.sub.c used for decoding the pulses and is therefore intended to compensate the time required for verifying the coding of the interrogation. The pulses thus validated then pass to a circuit 11 providing a complementary delay R.sub.co, which added to the compensation delay, gives the systematic delay .tau.: EQU .tau.=R.sub.c +R.sub.co
At the present time, different solutions exist to the problem arising from the high accuracy required for decoding the interrogations pulses and for the systematic delay .tau.. This accuracy must be greater in the "precision" P type interrogations but the solutions which will be described hereafter and which are suitable for type P should also be suitable for type N.
Considering the accuracy required for the systematic delay, on the one hand:
One of the solutions used at the present time consists in delaying the interrogation pulses by means of a multistage shift register, formed from a number N of series mounted storage circuits controlled by the same clock, i.e. a periodic signal of period T.sub.e. The maximum delay obtained with N stages is equal to N.T.sub.e with a resolution equal to T.sub.e. Since the delay thus obtained is programmable, so formed from a variable number of stages, registers must be used constructed either in accordance with the so called T.T.L. (Transistor-Transistor-Logic) technology or in accordance with the so called M.O.S. (Metal Oxide Semiconductor) Technology.
The drawbacks of this solution come from the limitations proper to the two technologies. The accuracy required for interrogations of type P (+ or -30 ns) implies a high block frequency. Now, in MOS technology, the frequency does not exceed 20 MHz and so does not allow the required resolution to be obtained. In so far as the value of the delay R.sub.c to be provided is concerned, it entails a considerable number of interrogator cases each forming a register, particularly in TTL technology.
Another solution which is also known consists of using a delay circuit formed from chains of counters disposed in parallel. The efficiency of such a system, the number of pulses at the output with respect to the number of pulses at the input, depends on the number of counters connected in parallel. The limitation of the number of chains causes a tendency to saturate-P, the delay circuit which then quickly becomes unavailable, all the more so if the multipaths or other interrogations of different codes are numerous.